Juha-Matti Tilli discovered a flaw in the way the Linux kernel
handled reassembly of fragmented IPv4 and IPv6 packets. A remote
attacker can take advantage of this flaw to trigger time and
calculation expensive fragment reassembly algorithms by sending
specially crafted packets, leading to remote denial of service.

This is mitigated by reducing the default limits on memory usage
for incomplete fragmented packets. The same mitigation can be
achieved without the need to reboot, by setting the sysctls:

Jann Horn discovered that the inode_init_owner function in
fs/inode.c in the Linux kernel allows local users to create files
with an unintended group ownership allowing attackers to escalate
privileges by making a plain file executable and SGID.

For Debian 8 Jessie, these problems have been fixed in version
4.9.110-3+deb9u2~deb8u1. This update includes fixes for several
regressions in the latest point release.

The earlier version 4.9.110-3+deb9u1~deb8u1 included all the above
fixes except for CVE-2018-5391,
which may be mitigated as explained above.

We recommend that you upgrade your linux-4.9 packages.

Further information about Debian LTS security advisories, how to apply
these updates to your system and frequently asked questions can be
found at: https://wiki.debian.org/LTS